Reflexes achieve their fixed nature by means of simple wiring. The sensory nerves that detect the stimulus (e.g. light on the the retina) are connected directly, or at worst via a miserly few intermediate neurones, to the motor nerves that bring about the reflex. This is the 'reflex' arc for the stretch reflex (below): the sensory neurones in the muscle connect directly to the motor neurones - no integration, no time wasting.
This contrasts with the normal route of sensory data (e.g. light on the retina), which usually gets taken up to the brain to get integrated, processed (e.g. into image of brother), and often consciously thought about. Only after all this computation do we react, if at all.
So much for the how. But why have reflexes at all? Surely it's better to hand over any sensory data to that mega-centre of intelligence, the brain, rather than to stab blindly about in the dark?
Well, the answer is that sometimes our responses simply can't wait that long. Though the whole process might seem instantaneous to you, it takes time for the messages from the sensory nerves to get to the brain, time for the brain to analyse it and decide what to do, and time to send the message back the various muscles. Depending on the type of nerve under consideration, 'conduction velocity' can be from around 10 to 100 metres per second, which, although fast, isn't always fast enough. Reflexes save this time, at the expense of only having a stereotypical repertoire.
Another reason for having a reflex is simply that we don't ever need to integrate and ponder certain things. Subjects like blood pressure control or temperature regulation are difficult even for the brightest of students, so it's just as well that these are partly controlled by reflexes.
Lastly, let's briefly glance at some of the commoner reflexes:
- Stretch reflex (or myotactic reflex) - when muscles are stretched, they contract so as to oppose this perturbation. One of this reflex's many uses is to maintain maintain posture. The muscles of the neck and back tend to be flexed by gravity, but this lengthening of the muscles is opposed by the stretch reflex. When we lean backwards or forwards at a slight angle, the gravity affects the muscles differently, but the reflex can respond quickly enough to keep us upright. The stretch reflex is most dramatic when the muscle is rapidly stretched, and this is the basis for the famous 'patellar tendon' reflex, where the doctor taps the area just below the kneecap with his hammer. The patient involuntarily jerks his leg straight, since the doctor has hit, and thus stretched, the tendon of the muscles that extend (straighten) the knee, eliciting the reflex.
- Flexor reflex - this is the rapid withdrawal of our limbs when encountering a painful stimulus. Remember that time you touched something very hot by accident, and recall how quickly you pulled your hand away... Again, consider the advantages of not waiting for the messages to go all the way to and from the brain before you could act!
- Pupillary constriction reflex - when bright light enters our eyes, our pupils constrict, aiming to keep the amount of light hitting our retina roughly constant. Too much or too little light, and our vision suffers.
- Corneal reflex - when you touch the cornea (the transparent bit in front of your pupil), you can't help but blink. This is trying to protect the must vulnerable part of your eye from injury, as blinking shields the eye and washes away irritants. Compare the corneal reflex (virtually impossible not to blink) with touching the sclera (the 'white' of the eye), where it is certainly possible not to blink.
- Gag reflex - most children have experimented with touching the back of the throat to elicit a gag. Why would the body have such a silly way of reacting to this? Well, the gag reflex is actually the start of what happens normally during eating. The food touches the back of the palate and pharynx, prompting you to swallow. And the start of swallowing is the gag reflex. The silly fun that you get from the it is because you are initiating the body's 'swallow' protocol... with your mouth open. Idiot.
And so on. There are many, many reflexes - wherever you don't need to think, or wherever thinking would be costly, a reflex could find a nice home.